The vendor doesn’t have any idea what he is talking about. If you disable BurnProof when you are already having buffer underrun symptoms, all this will do is create an unrecoverable buffer underrun and an automatic coaster. The BurnProof protection on the 16x Plextor does not affect burning speed.
If this vendor has given you any other advice, I would reexamine it to make sure that it is not part of your problem.
From your previous edited post, it appears that you have found that DMA is not enabled for the Plextor drive. If so, as we have already said, that is the problem, not BurnProof. If this same configuration ran fine in Win98 but not Win2000, it seems certain that you are having a DMA allocation problem in Win2000.
Check in your system BIOS and make sure that DMA mode is available for the Plextor, and that it is not set to PIO mode 4. Report the Plextor BIOS setting and what settings are available.
Check the slave setting on the primary channel as well to make sure that it is set to auto-detect or the actual capability of the drive.